DE 198 41 415 A1 discloses a film punch suitable for cutting plastic films/foils by means of a knife and a counter knife. A lift drive including a linear motor is provided. The lift drive includes a means of an articulated lever arrangement in combination with a straightening lever arrangement comprising together a large number of mechanical levers. Forces which are effective for cutting the film can increase rapidly and towards the end of the punching stroke, the increase of these forces becomes less steep. This is advantageous insofar as the film punch is intended to be suitable for reliably cutting different film/foil thicknesses without complicated knife adjustment operations being necessary.
The prior art discloses a film punch, type FS 08, produced by the applicant's company (see FIG. 1), which comprises a pneumatic cylinder as a lifting mechanism drive and which cuts through a film web by means of a squeeze knife and a counter-pressure bar. In order to reduce the load on and the wear of the squeeze knifes as far as possible when they come into contact with the counter-pressure bar and in order to be nevertheless able to cut through thick films/foils, the counter-pressure bar is provided with a fine adjustment device so that the counter-pressure bar can be adjusted with an accuracy of at least 0.02 mm relative to the end position of the squeeze knife.
As the squeeze knife is operated over time, the cutting edge of the squeeze knife dulls and the counter-pressure bar and/or the squeeze knife must be readjusted in order to guarantee a reliable, high-quality cut through of the film so as to ensure reliable processing. When such a film punch is used on thermoform packaging machines, the squeeze knifes, and perhaps also the counter-pressure bar, are often replaced in the case of a change of format, and, subsequently, the counter-pressure bar and the squeeze knife, respectively, have to be newly adjusted. This is done in a first test phase, with the thermoform packaging machine running, and requires a certain effort until cutting or cutting through of the film is accurately adjusted.
The load on the squeeze knife caused by the contact with the counter-pressure bar during each cutting or squeezing process may be very high, as can be seen from the time-force diagram according to FIG. 2. In order to be able to establish this kind of diagram, pressure sensors were provided in the area of the fine adjustment device in an experimental set-up, so as to detect the load on the counter-pressure bar and the squeeze knife, respectively. The diagram shows that, after cutting through of the film at the moment in time t2, the force increases still further, since the pneumatic cylinder with its force, which can be generated through the pressure applied, presses the squeeze knife and the counter-pressure bar against one another. In the exemplary case shown, this force F1 is more than twice as strong as would be necessary for the cutting process. A fundamental reduction of the pressurized air supply to the pneumatic cylinder is, however, not desired, since, if the pressure were for example halved, this would also lead to a reduction of the speed of the lift drive, whereby the performance per work cycle of the film punch would be reduced. The reduced speed with which the squeeze knife impacts the film also has a negative influence on the cutting quality when the film is being cut through.